Known infrared imaging systems typically include a planar detector area, known as the focal plane array, which consists of a plurality of planar detector pixels on a planar substrate that typically includes a read-out integrated circuit. The pixels are thermally isolated from, but electrically connected to, the substrate by way of mechanical isolation legs. The pixels act as micro-bolometers, in that infrared energy from the scene changes the pixel temperature, which further changes the pixel resistance. For each pixel, the change in resistance across the isolation legs is detected, measured, and represented by support circuitry, both in the substrate and other support circuit boards, to generate an infrared image.
For most infrared imaging systems, there are several characteristics that are extremely desirable. Specifically, it is desired that the systems have the best sensitivity achievable. It is also desirable that the infrared images generated have a high resolution for any given field of view. It is also generally desirable that infrared imaging systems have a larger field of view if task-required resolution can be retained. The shape of the pixels, as well as the arrangement thereof within the focal plane array, can affect these attributes, in that a field of view increase requires an increase in the number of pixels in the focal plane array and their closer location to each other if resolution and performance are to be retained.
The focal plane array unit cell fill factor is the ratio of active absorption area to unit pixel cell size. Unit cell fill factor can influence how the detector pixels are arranged on the focal plane array and the number of pixels per unit area. Different pixel shapes and different pixel arrangements could increase the unit cell fill factor and also fill some of the non-imaging real estate of the focal plane array with active absorption area. Such arrangements would provide increased resolution for the imaging device, and would improve infrared imaging performance in micro-bolometer based systems using single or multiple layer pixel designs.
In known infrared devices, the micro-bolometer pixels usually have a rectangular planar absorption area and are arranged in straight, perpendicular rows and columns. Further, the isolation legs typically extend outwardly from the pixel perimeter for a single layer structure, or are folded under the absorption area in right angle traces for a multiple layer structure. The rectangular shape of the current pixel and isolation leg structure would not greatly benefit from a staggered pixel arrangement in terms of focal plane array fill percentage. Although a staggered row or column design might help infrared imaging system performance with rectangular pixels in terms of image sampling, it would be of further benefit to change the shape of the pixel and allow them to be placed more closely together on the focal plane array.
In some instances, it is desirable to have an increased field of view for the infrared imaging system, which could result in increased angles of incidence for the incoming infrared radiation energy from the scene depending on the system front end optics. Absorption, and therefore sensitivity, could be improved if the incoming radiation is as orthogonal as possible to the pixel surface.
Superimposed over these considerations is the fact that micro-bolometer pixels are coincident with spaced-apart parallel planes, which establishes a tuned resonant cavity for the wavelength of interest between the focal plane array and the substrate for the device. During operation, a portion of the incoming infrared radiation passes through the pixel absorption surface and reflects off of the substrate to be absorbed again by the pixel. If pixel shapes are changed, then the resonant cavity efficiency of operation could be changed accordingly and perhaps adversely.
One way to increase pixel absorption, especially at increased angles of incident radiation, would be to step the absorption surface area of each micro-bolometer pixel in one or both directions. A method of decreasing the distance between pixels on the focal plane array would be to maximize the unit cell fill factor and pursue alternative shapes to the current rectangles.